Frangula alnus (Alder Buckthorn, also known in North America as Glossy buckthorn) is an ornamental woody species invading North American forests and prairies. De Kort et al. propose that repeated introductions from European source populations into North American parks facilitated the invasion of natural areas through providing stable and genetically diverse park populations.

Map showing geographical locations of populations in the invaded range (A) and the native range (B) of Frangula alnus. The symbols in the small box of panel (B) represent the three genetic clusters in this area. A more detailed picture of this box is provided in panel (C), which focuses on populations from northern France and Belgium, in Europe. Circle diagrams represent mean population probabilities of belonging to a genetic cluster, with different colours representing the 15 global genetic groups.

Using annotated genetic markers (SNPs), they show that multiple introductions from a limited number of European port regions to American parks led to the establishment of bridgehead populations with high allelic richness, allowing for continuous secondary migration to natural areas as well as rapid allele frequency shifts at putatively adaptive SNPs.